mm: expland documentation over __read_mostly
Subsystems (2)
PATCH v2 1/1 mm: expland documentation over __read_mostly
__read_mostly can easily be misused by folks, its not meant for just read-only data. There are performance reasons for using it, but we also don't provide any guidance about its use. Provide a bit more guidance over it use.
Signed-off-by Luis R. Rodriguez <[email protected]>
--- include/linux/cache.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
include/linux/cache.h +10 -2
@@ -15,8 +15,16 @@/* * __read_mostly is used to keep rarely changing variables out of frequently -* updated cachelines. If an architecture doesn't support it, ignore the -* hint. +* updated cachelines. Its use should be reserved for data that is used +* frequently in hot paths. Performance traces can help decide when to use +* this. You want __read_mostly data to be tightly packed, so that in the +* best case multiple frequently read variables for a hot path will be next +* to each other in order to reduce the number of cachelines needed to +* execute a critial path. We should be mindful and selective of its use. +* ie: if you're going to use it please supply a *good* justification in your +* commit log. +* +* If an architecture doesn't support it, ignore the hint. */ #ifndef __read_mostly#define __read_mostly
Signature · 1 line
2.17.0
Quoted 8 lines · Luis R. Rodriguez
__read_mostlycan easily be misused by folks, its not meant for just read-only data. There are performance reasons for using it, but we also don't provide any guidance about its use. Provide a bit more guidance over it use.Signed-off-by Luis R. Rodriguez <[email protected]>
After 2 years, this patch was never applied... and so people can easily keep misusing this. I'll resend now.
Luis
Quoted 30 lines
--- include/linux/cache.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)include/linux/cache.h +10
-3@@ -15,8 +15,16 @@/* * __read_mostly is used to keep rarely changing variables out of frequently -* updated cachelines. If an architecture doesn't support it, ignore the -* hint. +* updated cachelines. Its use should be reserved for data that is used +* frequently in hot paths. Performance traces can help decide when to use +* this. You want __read_mostly data to be tightly packed, so that in the +* best case multiple frequently read variables for a hot path will be next +* to each other in order to reduce the number of cachelines needed to +* execute a critial path. We should be mindful and selective of its use. +* ie: if you're going to use it please supply a *good* justification in your +* commit log. +* +* If an architecture doesn't support it, ignore the hint. */ #ifndef __read_mostly#define __read_mostly --2.17.0